1. Field of the Invention
The invention relates to a method for calibrating a diagnostic measuring device for biological signals, which can be represented as n-dimensional vectors. In medical diagnostics, such measuring devices are for example known in electroencephalography or in all fields of cardiography and vector cardiography and cardiogoniometry.
2. Description of Related Art
Such measuring devices and their diagnostic significance are, for example, based on measuring the electrical activity of an organ, with this activity differing in the healthy and sick state. In the case of the heart, cardiography is based on an electric field generated by membrane flows in the myocardial cells. The (sum) vector of this electric field generated by the heart varies over time in respect of its magnitude and its spatial orientation. The cardiac cycle, i.e. the electric progression of each heartbeat, can be subdivided into various segments. In a conventional electrocardiogram, the P-wave corresponds to the atrial excitation, the R-wave corresponds to the ventricular depolarization and the T-wave corresponds to the ventricular repolarization.
In cardiogoniometry, as described in EP 0 086 429, the cardiac flows are captured in four mutually orthogonal projections using four thoracic electrodes close to the heart in order to measure the magnitude of the potentials and to locate these in space. EP 1 048 000 presents a development of this teaching, which entails a computer-assisted computational analysis for improved representation and interpretation of the measurement results.
By way of example, the aforementioned P-, R-, and T-waves are represented as P-, R-, and T-vector loops in the spatial representation of vector cardiography or cardiogoniometry. These vector loops represent the path that the tip of the electric field vector generated by the heart passes over during the time of one heartbeat. The sum vector of the electric field generated by the heart over time runs over three loops in 3D space. The origin of the sum vector can be imagined to be a null point of a coordinate system for this space. This null point has to be defined because different measured values arise depending on the selection of the null point.
Firstly, the null point should correspond as closely as possible to a physiologically based null value and secondly it should be able to be established reliably despite the variability in the myocardial activity as a result of individual differences, a state of exertion, medical condition, etc. Moreover, various interfering influences such as e.g. offset voltages have to be filtered out. In such a physiological system, it is accordingly difficult to find a reliable null point as a reference point for calibrating the measuring device.